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[CSS21] Appendix E imprecise about background propagation to canvas

From: Anton Prowse <prowse@moonhenge.net>
Date: Fri, 20 Jul 2012 16:40:33 +0200
Message-ID: <50096DE1.2050508@moonhenge.net>
To: "www-style@w3.org" <www-style@w3.org>
E.2 says:

   # The canvas is transparent if contained within another, and given a
   # UA-defined color if it is not. It is infinite in extent and
   # contains the root element. Initially, the viewport is anchored with
   # its top left corner at the canvas origin.
   #
   # The painting order for the descendants of an element generating a
   # stacking context (see the 'z-index' property) is:
   #
   # 1. If the element is a root element:
   #    1. background color of element over the entire canvas.
   #    2. background image of element, over the entire canvas, anchored
   #       at the origin that would be used if it was painted for the
   #       root element.

and E.3 (Notes) says:

   # The background of the root element is only painted once, over the
   # whole canvas.


This fails to account for the special situation for the HTML BODY 
element described in 14.2:

   # The background of the root element becomes the background of the
   # canvas and covers the entire canvas, anchored (for
   # 'background-position') at the same point as it would be if it was
   # painted only for the root element itself. The root element does not
   # paint this background again.
   #
   # For HTML documents, however, we recommend that authors specify the
   # background for the BODY element rather than the HTML element.
   # For documents whose root element is an HTML "HTML" element or
   # an XHTML "html" element that has computed values of 'transparent'
   # for 'background-color' and 'none' for 'background-image', user
   # agents must instead use the computed value of the background
   # properties from that element's first HTML "BODY" element or XHTML
   # "body" element child when painting backgrounds for the canvas, and
   # must not paint a background for that child element. Such
   # backgrounds must also be anchored at the same point as they would
   # be if they were painted only for the root element.

I don't think we need to change the wording of the algorithm in E.2 
here, since 14.2 is clear about how it overrides the normal painting 
order.  But I do think that E.2 should acknowledge 14.2.  I propose 
doing this in the following explicit way.

Proposal A:

In E.2, prepend the following existing prose:

   # The painting order for the descendants of an element generating a
   # stacking context (see the 'z-index' property) is:
   #
   # 1. If the element is a root element:
   #    1. background color of element over the entire canvas.
   #    2. background image of element, over the entire canvas, anchored
   #       at the origin that would be used if it was painted for the
   #       root element.

with:

   | The background color and image of a child element of a root
   | element is sometimes painted over the entire canvas instead of that
   | of the root element itself.  This happens _in some situations with
   | HTML and XHTML documents_<link to 14.2>.  When this happens, no
   | background is painted for that child element.  User agents must
   | apply these rules instead of the ones below whenever the
   | latter concern the elements and backgrounds in question.

In E.3 (Notes), delete:

   # The background of the root element is only painted once, over the
   # whole canvas.


Note: the editorial device used in this proposal (that of explicitly 
discussing background propagation prior to the painting order algorithm 
whilst leaving the algorithm written in a manner which focuses the more 
typical general case) is deliberate, since it aids the proposal for a 
related issue in [1].


[1] https://www.w3.org/Bugs/Public/show_bug.cgi?id=17782

Cheers,
Anton Prowse
http://dev.moonhenge.net
Received on Friday, 20 July 2012 14:41:08 GMT

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